This invention relates to a method of and a device for tone display, and more particularly to a method of and device for tone display providing suitable brightness of displayed pictures, that is, fine tone display of brightness in display devices of pictures, etc., by controlling the luminous times of luminous elements corresponding to the pixels.
As a brightness control method of luminous elements corresponding to the pixels, there is a known method of tone display involving controlling the luminous times of the luminous elements. For example, a memory type plasma display is described in "A Proposal of the Drive Method for TV Using AC Type Plasma Display Panel", T. Kaji et al., ITEJ Technical Report Vol. IT72-45 (1973-03), pp. 1-10. This method uses a multiple-bit code to represent a fixed luminous time (say, one field of television signals, i.e. 1/60 sec.) To be more specific, time is assigned to the bits of a multiple-bit code in proportion to bit weights, and light emission or no light emission at the respective bits of the multiple-bit code of brightness is selected for each pixel, thereby controlling the gradation of brightness. Literature about television using tone display by time division includes "A Color TV Display Using 8-Inch Pulse Discharge Panel with Internal Memory", H. Murakami et al., and "Recovery of Capacitive Power Loss in Townsend Discharge TV", A. Kohgami et al., ITEJ Technical Report Vol. 13, No. 58 (1989), pp. 7-12.
As shown in FIG. 13, Murakami et al., divide time corresponding to a field into eight sub-fields, which are equally spaced. The respective sub-fields are arranged to correspond to the bits (b7 to b0) of a multiple-bit code representing the brightness of each pixel. At least in a selected sub-field of a multiple-bit code, the luminous elements are made to emit light on the display screen for a time corresponding to the weight of the sub-field.
As presented in FIG. 14, Kohgami et al. divide time corresponding to a field into eight time domains (b0 to b7) such that the eight time domains have the lengths of times corresponding to the weights of the bits of a multiple-bit light emission code. Over the whole time of at least a selected sub-field of a multiple-bit code, the luminous elements are made to emit light for display. Incidentally, the lines of the bits of FIG. 13 are shifted by 1 H (horizontal scanning time) from one another, since pulses for selecting lines are emitted a little shifted line after line so that two lines are not selected at the same time.
It has been found that in the prior art mentioned above, a disturbance such as a flicker occurs in tone display when the displayed picture is large, or the brightness is high or there is some movement in the picture. More specifically, when the display method of FIG. 13 is used, since the luminous time in one field is short, the displayed picture is dark, and therefore, adequacy in tone display is not achieved. When the display method of FIG. 14 is used, no more than 120 scanning lines are displayed, detail of the picture is not obtained, so that an inferior quality in tone display results. However, it has been made clear that when those methods were used in a large-sized gas-discharge television receiver with a 40-inch class screen, pictures were displayed so that the highest brightness is more than 50 FL (.apprxeq.171 dc/m.sup.2), and as a result, a notable disturbance occurs in the tone display in moving pictures.
To be more specific, when a person's face is shown, for instance, a white streak runs along the cheek when the face moves. In other words, in a smooth tone display, when something moves in a moving picture a streak occurs in some place, as if a tone bit is missing, a peculiar phenomenon that does not appear with the display elements of a conventional cathode-ray tube, for example, and this gravely impairs the picture quality of television.